Silver powder

For the synthesis of bare GQDs, we used our bottom-up synthesis approach as described previously.41 ,42 To functionalize the synthesized bare GQDs with PEG, we modified the irradiated mixture as follows. A 0.25 wt% mixture of nickel oxide powder (Alfa Aesar, Ward Hill, MA, USA) and 1.25 wt% of PEG bis (3-aminopropyl) terminated (Sigma-Aldrich St Louis, MO, USA) in 98.5 wt% benzene (Sigma-Aldrich) was irradiated for 45 minutes with a 1,064 nm pulsed Nd:YAG laser ([Continuum Surelite II, San Jose, CA, USA] KDP doubling crystal, 10 Hz, 10 ns pulse width). The synthesized GQDs were separated from benzene and the precipitated nickel oxide by centrifugation at 9,030× g, following the dissolution in nano-pure water. Finally, the GQDs solution was purified by using dialysis bags.
For the synthesis of Ag-GQDs, a mixture of 0.25 wt% of silver powder (Sigma-Aldrich) and 1.25 wt% of PEG bis(3-aminopropyl) terminated (Sigma-Aldrich) in 98.5 wt% benzene (Sigma-Aldrich) was prepared and irradiated by pulsed laser, similar to the synthesis of bare GQDs. The synthesized Ag-GQDs nanoparticles were separated from benzene and dissolved in nano-pure water, and purified by using dialysis bags.

To fabricate the Ag-PDMS composites, the PDMS prepolymer (Sylgard 184) and silver powder, 2–3.5 micron, 99.9+% (Sigma Aldrich) were first dispersed by hand using a spatula and then mixed in a planetary mixer (Thinky ARE-250) for 3 minutes at 2000 rpm and degassed for 1 minute at 2200 rpm. Next, PDMS curing agent was added (respecting a 10:1 ratio between the prepolymer and curing agent) and incorporated into the mixture by stirring manually before mixing again for 1.5 minute at 2000 rpm and degassing for 30 seconds at 2200 rpm. The viscous paste that was obtained was then stored in a freezer at −24°C until usage. Prior to usage, the paste was manually stirred with a spatula and mixed for 1 minute at 2000 rpm and degassed for 30 seconds at 2200 rpm.